The present invention relates to a process for producing 2,2xe2x80x2,3,3xe2x80x2,5,5xe2x80x2-hexamethyl-(1,1xe2x80x2-biphenyl)-4,4xe2x80x2-diol (to be sometimes referred to as xe2x80x9cHMBPxe2x80x9d hereinafter) and a process for producing abifunctional phenylene ether oligomer having a phenolic hydroxyl group at each terminal by using HMBP. More specifically, it relates to a process for producing HMBP useful for applications such as an intermediate for electronic material and an agricultural chemical intermediate and to a process for selectively producing a bifunctional phenylene ether oligomer which is excellent in electric characteristics, toughness, compatibility with a thermosetting resin and molding-processability and has a predetermined number average molecular weight.
As a method for synthesizing a biphenol compound by oxidatively coupling a phenol compound with an oxygen-containing gas, there is a method in which a substance selected from the group consisting of hydroxides, carbonates and bicarbonates of alkali metals, as a basic substance, is used in an aqueous solution containing an anion surfactant in the presence of a copper catalyst (JP-A-51-43748). When the above method is applied to 2,6-dimethyl phenol represented by the formula (3), 3,3xe2x80x2,5,5xe2x80x2-tetramethyl-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diol (to be referred to as xe2x80x9cTMBPxe2x80x9d hereinafter) represented by the formula (4) can be obtained relatively at high yield. However, since the aqueous solvent containing the surfactant is strongly stirred (6,000-10,000 rpm), a problem is that intense foaming occurs. Further, the above method requires a special device. 
Further, there is known a method which does not require a high-speed rotation thanks to an addition of a boron compound as a pH controlling agent (JP-A-60-152433). In particular, this method is effective for an oxidative coupling of an easily oxidizable phenol having substituents at 2- and 6-sites.
Materials for use in an electric or electronic field are required to have low dielectric characteristic for processing high-volume data at high speed in the advanced information society and toughness for preventing the occurrence of microcracks due to thermal shock, etc. For the above requirements, the use of engineering plastic such as polyphenylene ether (to be sometimes referred to as xe2x80x9cPPExe2x80x9d hereinafter) is proposed.
PPE has excellent high-frequency properties. On the other hand, known problems of PPE are that it is poor in compatibility with a thermosetting resin such as an epoxy resin or a cyanate resin, that it has a high melt viscosity so that molding-processability is poor, and that a solvent in which it is soluble is limited to an aromatic hydrocarbons solvent such as toluene, benzene or xylene and an halogenated hydrocarbon solvent such as methylene chloride or chloroform so that workability is poor.
For improving the compatibility, a method of improving compatibility by blending PPE with a different resin as a compatibilizing agent is discussed and the pseudo IPN structuralization of PPE and a cyanate resin is also discussed (JP-A-11-21452, etc.). However, problems of molding-processability and heat resistance have not been solved yet. Further, a method of converting a high molecular PPE into a low molecular compound is discussed for improvingmoldability. For example, there are known a method in which a high molecular PPE and a bivalent phenol are redistributed in the presence of a radical catalyst (JP-A-9-291148, etc.) and a method in which a bivalent phenol and a monovalent phenol are subjected to oxidation polymerization (JP-B-8-011747). However, a polymer is present in each of the above methods so that it is impossible to efficiently obtain a bifunctional phenylene ether oligomer compound having a desired molecular weight.
It is an object of the present invention 1 to provide a process for producing biphenol (to be sometimes referred to as xe2x80x9cHMBPxe2x80x9d hereinafter) of the formula (2) at high yield without the generation of byproducts by oxidation coupling of alkyl phenol while inhibiting the generation of diphenoquinone.
It is an object of the present invention 2 to provide a process for selectively producing a bifunctional phenylene ether oligomer compound having a PPE structure and a desired number average molecular weight which oligomer compound has the excellent electric characteristics and toughness of PPE and is improved in compatibility with a thermosetting resin and molding-processability.
It is another object of the present invention to provide a process for selectively producing a bifunctional phenylene ether oligomer compound having a PPE structure and a desired number average molecular weight which oligomer compound is soluble in a general-purpose ketone solvent and has terminal phenolic hydroxyl groups which are easily modifiable.
According to the present invention 1, there is provided a process for producing HMBP of the formula (2), which process comprises,
while setting a pH of a reaction solution containing an alkaline aqueous solution, a surfactant, a copper catalyst and 2,3,6-trimethylphenol represented by the formula (1) during a reaction in the range of from 8 to 14 and controlling the variation range of the pH of the reaction solution within xc2x11,
oxidatively coupling the 2,3,6-trimethylphenol with an oxygen-containing gas. 
According to the present invention 2, there is provided a process for producing a bifunctional phenylene ether oligomer compound having a controlled average molecular weight, represented by the formula (5), comprising carrying out oxidation polymerization at an oxygen concentration lower than an oxygen concentration of an explosion limit of a solvent to be used by charging a reactor with a mixture containing a catalyst, an amine and a solvent in advance and then dropwise adding a mixture containing HMBP of the following formula (2), a monovalent phenol of the following formula (6), an amine and a solvent to the mixture in the reactor, thereby producing a bifunctional phenylene ether oligomer of the formula (5),
wherein, from a moment when the number average molecular weight of the bifunctional phenylene ether oligomer compound exceeds 500, the supply rate of the dropwise addition mixture is controlled such that the concentration of the monovalent phenol of the formula (6) in the reactor does not exceed 15% by weight based on the total amount of the above monovalent phenol, the bifunctional phenylene ether oligomer and the HMBP [(6)/(5)+(2)+(6) less than 0.15], 
in which R1 and R2 may be the same or different and are a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group, R3 and R4 may be the same or different and are a hydrogen atom, a halogen atom, an alkyl group having 6 or less carbon atoms or a phenyl group, and each of m and n is an integer of 0 to 25 while at least one of m and n is not 0.
According to the present invention 2, further, there is provided a process according to the above, wherein the mixture containing the catalyst, the amine and the solvent charged in the reactor in advance further contains HMBP.
According to the present invention 2, further, there is provided A process according to the above, wherein the molar ratio between the HMBP of the formula (2) and the monovalent phenol of the formula (6) is 1:1 to 1:15.